Hot melt adhesives are solid at room temperature but, upon application of heat, melt to a liquid or fluid state. The hot melt adhesive is applied onto substrates during this liquid or fluid state. On cooling, the adhesive regains its solid form. The hard phase(s) formed upon cooling the adhesive imparts all of the cohesion (strength, toughness, creep and heat resistance) to the final adhesive. “Reactive” or crosslinkable hot melt adhesives, which are also applied in molten form, cool to solidify and subsequently cure by a chemical crosslinking reaction. An advantage of hot melt curable adhesives over traditional liquid curing adhesives is their ability to provide “green strength” upon cooling prior to cure. Advantages of hot melt curable adhesives over non-curing hot melt adhesives include improved temperature and chemical resistance.
The majority of “reactive” or crosslinkable hot melts are based on moisture-curing urethane adhesives. These adhesives consist primarily of isocyanate terminated polyurethane prepolymers that react with surface or ambient moisture in order to chain-extend, forming a new polyurethane/urea polymer. Polyurethane prepolymers are conventionally obtained by reacting polyols with isocyanates. Cure is obtained through the diffusion of moisture from the atmosphere or the substrates into the adhesive, and subsequent reaction. The reaction of moisture with residual isocyanate forms carbamic acid. This acid is unstable, decomposing into an amine and carbon dioxide. The amine reacts rapidly with isocyanate to form a urea. The final adhesive product is an irreversible material polymerized primarily through urea and urethane groups.
There are several drawbacks to these “reactive” hot melt compositions. The materials are typically made by reacting excess diisocyanate with polyols. The compositions usually contain two to five percent of unreacted diisocyanates such as MDI (methylene bisphenyl diisocyanate), which are respiratory and skin sensitizers. Also, moisture curable reactive hot melt compositions must be stored in the absence of moisture to prevent premature curing, typically stored under nitrogen, or under inert gas blankets in expensive packaging such as polyolefin coated aluminium bags, to prevent moisture ingress. Moreover, moisture curable reactive hot melt adhesives require costly specialized application equipment for processing to keep moisture out. Furthermore, cure rates of such reactive hot melt composition are variable and highly dependent upon moisture permeating through the adhesive. Hence, atmospheric humidity, moisture content of the substrates, moisture vapor transmission rate of the adhesive and the thickness of the bond line all contribute to the cure rate of reactive hot melt compositions. In addition, polyurethane moisture cure produces CO2, which can cause bubbles in the bond line, weakening the bond strength, and which can cause aesthetic problems, particularly on plastic substrates. Additionally, reactive hot melts cure irreversibly and hence reworkability or remendability is not feasible. Such adhesives cannot be used if premature curing occurs or they cannot be repositioned and/or removed after cure has taken place.
There remains a need for improvements in reactive hot melt technology to expand the application of such adhesives and their effectiveness in such applications. The present invention addresses this need.